The present invention relates to a process for preparing a polycarbonate from a dialkyl oxalate and a phenolic compound.
A process for preparing a polycarbonate from a dialkyl oxalate and a phenolic compound which comprises a first step of transesterifying the starting compound to produce a diaryl oxalate, a second step of decarbonylating the resulting diaryl oxalate to produce a diaryl carbonate, and a third step of subjecting the diaryl carbonate to condensation-polymerization reaction with a polyvalent hydroxyl compound to produce a polycarbonate is described in Japanese Patent Provisional Publication No. 10-152552. The Publication 10-152552 further describes such procedure that a phenolic by-product produced in the condensation-polymerization reaction is recovered and recycled to employ as the phenolic compound in the first step.
In the above-mentioned process, the diaryl carbonate of a high purity can be produced using no troublesome phosgene. Therefore, the diaryl carbonate produced from a dialkyl oxalate and a phenolic compound is favorably employed to react with a polyvalent hydroxyl compound to give a polycarbonate having high quality such as high transparency through condensation-polymerization reaction.
It is also known that a catalyst containing tetramethylammonium hydroxide, tetrabutylammnium hydroxide, or tributylamine is employed as a catalyst for performing a condensation-polymerization reaction for the preparation of a polycarbonate from a diaryl carbonate such as diphenyl carbonate and a polyvalent hydroxyl compound such as bisphenol A.
It has been discovered that a portion of the above-mentioned condensation-polymerization catalyst decomposes at an elevated temperature to produce an amine compound (by-product) such as trimethylamine or tributylamine. The amine by-product is migrated into the recovered phenolic by-product.
According to the further study of the investors, in the case that the phenolic by-product is employed as the phenolic compound in the preparation of the diaryl oxalate, the amine by-product in the phenolic by-product gives adverse effects (e.g., deactivation of the catalyst employed) to the reactions in the steps of transesterification and decarbonylation if the amount of the amine by-product in the phenolic by-product increases.
It has been further discovered that the phenolic by-product recovered in the condensation polymerization reaction is sometimes contaminated with an ethereal real by-product. The ethereal by-product in the phenolic by-product sometimes gives such disadvantageous effects as to disturb recovery of reactants and/or reaction product from the reaction system, if the amount of the ethereal by-product in the phenolic by-product increases.
The present invention resides in a process for preparing paring polycarbonate which comprises the steps of:
subjecting a dialkyl oxalate and a phenolic compound to transesterification to give a diaryl oxalate;
subjecting the diaryl oxalate to decarbonylation to give a diaryl carbonate;
reacting the diaryl carbonate with a polyvalent hydroxyl compound in the presence of a catalyst containing an amine compound or an ammonium compound to give the polycarbonate, removing a mixture containing a phenolic by-product and an amine by-product;
collecting the mixture;
purifying the mixture so that the amount of the amine by-product in the mixture is reduced to give a phenolic by-product mixture not containing the amine by-product in an amount of more than 600 ppm; and
utilizing thus purified phenolic by-product mixture as a whole fir a part of the phenolic compound give the first step.
In the above-mentioned process, if the collected mixture further contains an ethereal by-product, the mixture is preferably purified to reduce its ethereal by-product content to a level of not more than 600 ppm.
The invention further resides in a process for preparing polycarbonate which comprises the steps of:
subjecting a dialkyl oxalate and a phenolic by-product mixture not containing an amine by-product in an amount of more than 600 ppm to transesterification to give a diaryl oxalate, said phenolic by-product mixture having beer obtained by purifying a mixture containing a phenolic by-product and an amine by-product which was collected in a condensation polymerization reaction for preparing polycarbonate from a dialkyl oxalate and a phenolic compound;
subjecting the diaryl oxalate to decarbonylation to give a diaryl carbonate; and
reacting the diaryl carbonate with a polyvalent hydroxyl compound in the presence of a catalyst containing an amine compound or an ammonium compound to give the polycarbonate.
In the above-mentioned process, if the phenolic by-product mixture further contains an ethereal by-product, the mixture is preferably purified in advance to reduce its ethereal by-product content to a level of not more than 600 ppm.
FIGURE is a schematic flow chart to show a process for preparing a polycarbonate according to the present invention.